The invention generally relates to high speed weighing equipment of the type employing a conveyor to transport containers one at a time across a vertically deflectable weigh pan or scale device operable to effect weighing of the products. More particularly, the invention relates to improvements in weighing equipment of the type intended for use in the weighing of containers having either circular or oval footprint configurations and employing conveyors characterized as having a pair of parallel conveying elements, such as endless chains, arranged to underengage the footprints of the containers as they transport the containers for movement across a weigh pan.
When products to be introduced to a checkweigher for weighing purposes are containers having circular or oval footprint configurations, it has been standard industrial practice to introduce the containers from an upstream operation, such as from a filler, in a butt-to-butt or adjacent container touching condition, and then employ a timing worm or other spacing device to accelerate the containers up to a given conveyor transport speed required by system operating conditions and to properly space containers lengthwise of the conveyor in order to ensure the presence of only one container on the weigh pan during each weighing operation.
In the design of checkweighing equipment of the type described, use may be made of the following equations to determine settle time, i.e. the time from when a container is fully on a weigh pan or scale platform until it begins to be transferred off the weigh pan: EQU Pitch=weigh pan length (ft.)+margin of safety EQU Speed=pitch.times.container feed rate EQU Settle time= ##EQU1##
An accepted shortcut to calculate settle time in milliseconds is: EQU t(ms)=5000.times.(wpl-pl)/v, EQU where t=time in ms EQU 5000=conversion factor for ft./in. and min./ms EQU wpl=weigh pan length in inches EQU pl=container length in inches EQU v=conveyor speed in fpm.
The type of weighing mechanism employed will determine the required minimum settle time which must be made available in order to weigh each container, with the accuracy of the results of the weighing operation tending to increase as the length of settle time increases beyond such minimum.
The length of the weigh pan was determined by trial and error upon consideration being given to container feed rate, container length and conveyor speed with weigh pan length increasing as these factors increase. The margin of safety, which is combined with weigh pan length to produce the required pitch or distance between leading edges of adjacent containers, is typically required to be on the order of one inch for the normal range of conveyor speeds in order to ensure against the presence of more than one container on the weigh pan during a weighing operation.
The length of the container is determined by its size and the requirement that the conveyor be designed to provide the most stability for the container as it is conveyed across the weigh pan, during which time it is not otherwise stabilized, as by side mounted guides whose presence would interfere with vertical movement of the container during the weighing operation. Typically, the selection of the distance between centers of the conveyor elements for best container support was determined for containers having circular footprints by a simple geometric construction, where mutually perpendicular diameters were drawn in a circle that represents the footprint of the container, and then two parallel chords were drawn connecting the ends of the diameters to represent the centers of the conveyor elements. The chords defined by the inner or facing edges of the conveyor elements underengaging the footprint, which are parallel to and inwardly of the drawn chords, were then used as the container length in solving the above settle time equation.
Checkweighers constructed with the above considerations in mind have proven adequate for many years to handle container feed rates commonly encountered in the food processing industry. In recent years, demands for checkweighers capable of handling higher container feed rates have been met by increasing weigh pan length and conveyor speed. However, as conveyor speeds have been increased, there has been experienced an increase in problems, including for example an increase in conveyor noise and wear, and reduction in stability of containers during a weighing operation, particularly for the case of taller containers.